1. Technical Field
The present invention relates to an electrodeless discharge lamp lighting device and a lighting apparatus.
2. Related Art
FIG. 21 illustrates a configuration of a conventional electrodeless discharge lamp lighting device. Reference numerals 50a and 50b are connection terminals suitable for connection to a power source. Reference numeral 50 denotes a current source for generating high-frequency pulse current from voltage the power source. An output terminal of the current source 50 is connected to an input terminal of a modulator 51 and the modulator 51 modulates the amplitude of the high-frequency pulse current to square-wave current having a predetermined duty cycle at a predetermined modulation frequency. An output terminal of the modulator 51 is connected to an input terminal of an amplitude adjusting circuit 52 for adjusting the amplitude of the square-wave modulated current. A low-pressure mercury discharge lamp La is connected between an output terminal of the amplitude adjusting circuit 52 and a ground terminal. Reference numeral 53 denotes a duty cycle adjusting circuit for adjusting a duty cycle and reference numeral 54 denotes a modulation frequency adjusting circuit for adjusting a modulation frequency. An output terminal of the duty cycle adjusting circuit 53 is connected to an input terminal of the modulator 51 and an output terminal of the modulation frequency adjusting circuit 54 is also connected to another input terminal of the modulator 51.
Accordingly, as well as the modulation frequency and the amplitude, the duty cycle of current in the shape of square-wave high-frequency pulse to be modulated can be adjusted, thereby enlarging the range of optical color point to be adjusted with a predetermined flux of light. At the same time, it is to rising the extent to which the position of the optical color point of light emitted from the lamp does not depend on the value of the flux of light from the lamp (for example, see Patent Document 1).
There is also known an electrodeless discharge lamp lighting device for controlling the modulation of light by varying the frequency of a voltage to be supplied to an electrodeless discharge lamp by the use of a light modulation adjuster (for example, see Patent Document 2).
[Patent Document 1] Japanese Patent No. 3092955
[Patent Document 2] Japanese Patent Laid-open No. 63-158796
The conventional discharge lamp lighting device disclosed in Patent Document 1 lights the discharge lamp of load by applying voltage to electrodes formed to a luminous body of the discharge lamp and electrolyzing gas in the luminous body. A circuit for such discharge lamp lighting device forms a current loop that an electric current flows from one output terminal in the circuit to one electrode of the luminous body via one lighting tube line, to another electrode of the luminous body via plasma in the luminous body, and to another output terminal of the circuit via another lighting tube line via.
Magnetic-component noises are generated by the current loop formed as described above, but it is sufficient that only the luminous body is externally visible. By covering most of the current loop, the magnetic-component noises can be blocked by shielding.
However, as disclosed in Patent Document 2, in the electrodeless discharge lamp lighting device having an induction coil for lighting an electrodeless discharge lamp, a magnetic field resulting from the induction coil is used as means for supplying power to the electrodeless discharge lamp. In this case, when the magnetic field is shielded with metal or the like, power cannot be supplied to plasma in the electrodeless discharge lamp, thereby making the lighting impossible. Moreover, the plasma in the electrodeless discharge lamp is formed in a loop shape and when most of the plasma in the loop shape is covered, light cannot be emitted externally, thereby deteriorating the optical output.
In such electrodeless discharge lamp lighting device having the induction coil, since the current loop cannot be covered with metal so as to suppress the magnetic-component noises, it is necessary to add a noise suppression part to the circuit elements, thereby causing increase in size and cost of the device.